1. Field of the Invention
The present invention relates to a piezoelectric displacement element, and more particularly, it relates to structure of a piezoelectric displacement element utilizing a sintered body obtained by stacking a plurality of piezoelectric ceramic layers in the direction of thickness and co-firing the same.
2. Description of the Prior Art
As roughly illustrated in FIG. 1 showing a side view, Japanese patent Laying-Open Gazzete No. 69999/1981 discloses a piezoelectric displacement element employing a sintered body which is obtained by stacking a plurality of ceramic layers 2a to 2f through a plurality of internal electrodes 1b to 1f between electrodes 1a and 1g and co-firing the same. The electrodes 1a and 1g are formed similarly to the electrodes 1b to 1f or separately after firing. For performing a polarization process in the piezoelectric displacement element according to this prior art, electrical connection is effected as shown in FIG. 1 followed by application of voltages, whereby the respective ceramic layers 2a to 2f are polarized in the directions shown by arrows in FIG. 1. After such polarization is performed, the plus side and the minus side as shown in FIG. 1 are electrically connected so as to apply alternate currents from left and right external electrodes, whereby a first group of the ceramic layers 2a to 2c and a second group of the ceramic layers 2d to 2f are expanded and contracted in reverse directions to each other, so that the entire piezoelectric element causes bending movement. Thus, although such a multi-layered type piezoelectric displacement element operates as a whole in a similar manner to a well-known bimorph vibrator, the thickness of the respective ceramic layers 2a to 2f can be considerably reduced since the sintered body utilized therein is obtained by stacking the ceramic layers 2a to 2f and co-firing the same. Therefore, the impedance can be lowered and the amount of displacement may readily be increased. However, since, in the piezoelectric displacement element having the aforementioned construction, the piezoelectric ceramic layers 2a to 2f are co-fired, the internal electrodes 1b to 1f are generally formed by coating metal paste or the like for forming the electrodes on the ceramic layers 2a to 2f and burning the same. Thus, the internal electrodes 1b to 1f are considerably thin in thickness, e.g., about 2 .mu.m to 5 .mu.m. Therefore, when the piezoelectric displacement element as shown in FIG. 1 is driven, ceramic crystal particles of the central ceramic layers 2c and 2d are expanded and contracted in reverse directions to each other, whereby the piezoelectric ceramic particles on both sides of the internal electrode 1d reciprocally constrain mutual expansion and contraction. In other words, since the first group of ceramic layers 2a to 2c provided above the internal electrode 1d and the second group of ceramic layers 2d to 2f provided below the same are displaced in opposite directions, the stress from the piezoelectric ceramic layers 2c and 2d provided on both sides of the internal electrode 1d concentrates in the vicinity of the internal electrode 1d. Consequently, such stress concentration influences the displacement to cause considerably large displacement hysteresis in the piezoelectric displacement element as shown in FIG. 1. In other words, since the crystal particles forming the piezoelectric ceramic layers 2c and 2d through the internal electrode 1d are expanded and contracted in reverse directions with interposition of the internal electrode 1d, large displacement hysteresis is caused when positive and negative voltages are alternately applied.